Hydrogen (H2) gas sensors that are sensitive, rapid-responding, stable, compact, and inexpensive are needed to optimize the performance and insure the safety of devices like fuel cells that are powered by H2. Palladium (Pd) nanowires are attractive candidates for H2 sensors because they are able to equilibrate rapidly with H2, leading to a rapid response time. Palladium absorbs hydrogen to form a hydride (PdHx) with x saturating at 0.67 and since 1869 it has been known that the electrical resistivity of this hydride increases linearly with x by a factor of 1.8-1.9 over the range from x=0 to 0.67 (see, Lewis, Academic Press, 1967; Flanagan, Annual Reviews 1991, 21. 269-304). This property of PdHx was first exploited for hydrogen sensing by Hughes and Schubert (J. Applied Physics 1992, 71, 542-544). As hydrogen gas sensors, Pd resistors are elegant in their simplicity but they have several deficiencies: 1) Hydrogen atom diffusion in palladium is slow at room temperature (DH=3.8×10−7 cm2/s at 298K). This means that the Pd resistor must be heated to 70° C. or higher to activate diffusion degrading the power efficiency of the device, 2) The alpha to beta phase transition of PdHx, occurring over the range from 1-2% H2, mechanically stresses the resistor causing deformation and delamination while simultaneously retarding the sensor response time, 3) hydrogen sulfide, ammonia, water, and hydrocarbons interfere with H2 detection at Pd because they dissociatively chemisorb to produce adsorbed hydrogen atoms.
By reducing the distance over which hydrogen must diffuse within the palladium sensing element, the retarding effect of slow proton diffusion on the response time of the resistor is minimized. Early sensors consisted of ensembles of hundreds of Pd nanowires, 150-300 nm in diameter. Exposure of these nanowires to H2 at concentrations above 2% caused each nanowire to fracture approximately every 2 μm along its axis resulting in a loss of electrical continuity (see, Favier, Science, 2001, 293, 2227-2231). Subsequent exposures to H2 above 1-2% threshold for the alpha to beta phase transition swelled the nanowire and closed these fractures, restoring electrical continuity. These sensors had a rapid response time of less than one second, but the limit-of-detection (LODH2) was in the 2% range necessary to induce the alpha to beta phase transition. This LODH2 is too high even for H2 leak detection since the lower explosion limit for H2 of 4% is just incrementally higher.
Since 2002, palladium nanostructures have been used in a variety of innovative ways as resistor-based hydrogen sensors. These sensors can be categorized according to the mechanism by which they transduce hydrogen: Sensors that derive their signal from the volume change associated with the alpha to beta phase transition generally show decreased resistance in the presence of hydrogen (i.e., ΔRH2 (−)) while those that measure the increased resistance of the PdHx relative to Pd show an increased resistance upon H2 exposure (i.e., ΔRH2 (+)). Two dimensional palladium nanoparticulate films fall into the first category. An attribute of these systems is that they often have rapid response times (<1 s) that mimic the early palladium nanowire arrays, but they are much easier to fabricate. Single electrodeposited palladium nanowires have been shown to function as H2 sensors in this ΔRH2 (−) mode (see, Yun, Small 2006, 2, 356-358). A ΔRH2 (−) sensors have also produced by using a focused ion beam to cut a nanotrench with width 100-400 nm into a palladium microwire (see, Kiefer, J. Nanotechnology, 2008, 19, 25502). With a few exceptions, ΔRH2 (−) sensors show a LODH2 in the 1-2% range coinciding with the threshold for the alpha to beta phase transition. A lower LODH2 can be obtained for systems capable of functioning in the ΔRH2 (+) regime because the increased resistance of PdHx can be detected well below the 1-2% threshold for the alpha to beta phase transition, often at the expense of slower sensor response and recovery times. Recently, a sensitive ΔRH2 (+) hydrogen sensor was obtained when carbon nanotubes arrayed between two electrical contacts were electrochemically decorated with palladium nanoparticles. These sensors showed a LODH2 of 100 ppm with response times in the 5-10 minute range (see, Myuang, J. Phys. Chem. 2007, 111, 6321-6327).
Therefore, it is desirable to have palladium nanowire hydrogen sensor with a LODH2 below 1-2% and an fast response time.